The present invention relates to a ceramic foam filter having an integral gasket and a method of forming the filter.
Molten metal, particularly molten aluminum, in practice generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as inclusions in the final cast product after the molten metal is solidified and cause the final product to be less ductile or to have poor bright finishing and anodizing characteristics. The inclusions may originate from several sources. For example, the inclusions may originate from surface oxide films which become broken up and are entrained in the molten metal. In addition the inclusions may originate as insoluble impurities, such as carbides, borides and others or eroded furnace and trough refractories.
Porous ceramic foam materials are known to be particularly useful in filtering molten metal, as described in U.S. Pat. No. 3,893,917 for "Molten Metal Filter" by Michael J. Pryor et al., patented Jul. 8, 1975, and also as described in U.S. Pat. No. 3,947,363 for "Ceramic Foam Filter" by Michael J. Pryor et al., patented Mar. 30, 1976 and U.S. Pat. No. 4,081,371 for "Filtering of Molten Metal" by John C. Yarwood et al., patented, Mar. 28, 1978, all of which are assigned to the assignee of the instant invention and incorporated herein by reference.
These ceramics foam materials are particularly useful for filtering molten metal for a variety of reasons included among which are their excellent filtration efficiency, low cost, ease of use and ability to use same on disposable, throwaway basis. The fact that these ceramic foam filters are convenient and inexpensive to prepare and may be used on a throwaway basis requires the development of means for easily and conveniently assembling and removing porous, molten metal filters from a filtration unit while providing a highly efficient filtration assembly. Since the filters are designed to be a throwaway item, it is essential to provide an effective means of sealing the filters in place in its holder which is easy to assemble, disassemble and clean up. The holder or filter chamber itself is normally an integral part of a trough, pouring pan or tundish etc. and should be constructed of refractory materials resistant to the molten metal similar to those used in standard trough construction.
It is greatly preferred to seal the filter plate in place using a resilient sealing means or gasket type seal peripherally circumscribing the filter plate at the bevelled portion thereof. The gasket type seals ensure a leak free installation and also provide an effective parting medium which is essential for ease of disassembly. In addition, since the gasket or sealing means prevent ingress of metal to the sealing faces of the holder unit, their use considerably eases clean up and effectively prolongs the life of the unit by eliminating problems of metal attack. Furthermore, because of its resiliency, the gasket may provide sufficient frictional force to hold the filter body in place in the holder or filter chamber without resorting to other types of hold down devices. The resilient sealing means should be non-wetting to the particular molten metal, resist chemical attack therefrom and be refractory enough to withstand the high operating temperatures.
Heretofore, the ceramic foam filters were gasketed with pre-cut strips of low density ceramic fiber having an organic binder. The straight pre-cut strips were simply taped to the bevelled edges of the filters after the filter had been baked and fired. As a result of the foregoing process, a number of disadvantages arose. These disadvantages include: sagging and detachment of the gasket from the filter as the tape loosens up and peels off; tearing of the gasket in handling due to the fragile nature of the material resulting in metal bypass; giving off of undesirable and unhealthy fumes during pre-heating, due to the burn off of organic binders in the material, including carbon monoxide, carbon dioxide, hydrocarbons and formaldehyde; regassing of the melt due to binder gases which are given off; less than desirable appearance; and high economic cost due to the labor necessary to cut, fit and tape the gasket manually. Yet other disadvantages include the occurrence of misfits due to the pre-cut strips being straight and the filter edges being bevelled and the need to coat the filter edges with additional ceramic slurry in order to insure that the filter edges are not directly handled during further processing. Misfit is a particularly troublesome problem in that in some cases it results in the molten metal passing between the gasket and the filter plate.
Another gasketing technique for ceramic foam filter comprises applying an expandable gasketing material having an adhesive peel-off backing to the filter. The material is cut and fitted manually around the filter. One of the disadvantages of this approach is that the glue burns off along with the binders in the gasket and gives off undesirable fumes. Other disadvantages include: metal bypass of the joints due to poor fit and failure of the gasket to expand and seal the gap effectively; regassing of the melt where preheating is not done long enough at high enough temperature; loosening of the gasket material; possible mechanical failure of the filter due to uneven pressure exerted on the filter by over expansion of the gasket; and high economic costs associated with the gasket plus the labor involved in installing the gasket.
Attempts have been made to mold a gasket about a ceramic foam filter. U.S. Pat. No. 4,331,621 to Brockmeyer illustrates such an attempt. In accordance with this patent, a mold was provided having dimensions corresponding to the desired final filter size which is to be inserted in the filter bowl. A fired filter was placed in the center mold. The gap between the mold and filter was filled with gasketing material. After drying, the mold was taken off. These attempts failed because there was difficulty separating the mold from he gasket. There was also a lack of adherence of the gasket material to the filter.
Accordingly, it is an object of the present invention to provide a ceramic foam filter having a gasket material mechanically and chemically bonded thereto.
It is a further object of the present invention to provide a method of forming the desired gasketed filter while achieving utility, economical and aesthetic goals.
These and further objects and advantages will become more apparent from the following description and drawing in which like reference numerals depict like elements.